The present invention relates to a digital-to-analog converter circuit assembled into a successive approximation type analog-to-digital converter fabricated into a semiconductor integrated circuit.
This type of analog-to-digital converter requires a digitial-to-analog converter for generating analog signals are references, according to a digital signal. An example of this type of D/A converter is shown in FIG. 1. This circuit employs a string resistor network which is suitable for fabricating the circuit into an IC chip. In FIG. 1, a reference voltage Vref produced by a reference voltage source V is applied across a string of resistors r1 to rn connected in series. The divided voltages divided by the resistors r1 to rn are respectively connected to an analog output terminal Aout through switches S0 to Sn. One of the switches S0 to Sn is selected according to a digital input Din applied through a switch select logic circuit SEL. The number of the switches S0 to Sn is selected corresponding to the number of bits of the digital input Din. For this reason, if the number of bits of the digital input increases, the number of switches is increased, resulting in increase of the circuit size, and hence enlargement of the chip size. For example, for the digital input Din of 8 bits, 256 switches are required. For the digital input of 9 bits, 512 switches are required. 1024 switches are required for the digital input of 10 bits. Thus, as the number of bits of the digital input increases, the number of switches required greatly increases.
Generally, a high accuracy D/A conversion requires an increased number of bits of the digital input. This provides many problems. The increased number of switches increases the chip size. The increase of the chip size increases the cost to manufacture. The increased number of switches increases a stray capacitance associated with the switches. The increased stray capacitance slows the response of the analog output when the digital input Din changes, decreasing the operating speed of the device.